19) -8k³ + 125t³ =

Rewrite as a sum of cubes:

125t³ - 8k³

Apply the difference of cubes formula (a³ - b³ = (a - b)(a² + ab + b²)):

(5t)³ - (2k)³

(5t - 2k)((5t)² + (5t)(2k) + (2k)²)

(5t - 2k)(25t² + 10tk + 4k²)